Internal combustion engines include camshafts that open and close valves regulating the combustion of fuel and air within combustion chambers of the engines. The opening and closing of the valves are carefully timed relative to a variety of events, such as the injection of fuel into the combustion chamber for combustion, the location of the piston relative to top-dead center (TDC), and the rotational speed of the crankshaft to name a few. Camshaft(s) are driven by the rotation of the crankshaft via a drive member connecting these elements, such as a belt or chain.
In the past, a fixed relationship existed between the rotation of the crankshaft and the rotation of the camshaft. However, internal combustion engines increasingly use variable camshaft timing (VCT) to vary the phase of camshaft rotation relative to crankshaft rotation. VCT can be carried out by camshaft phasers that are actuated and controlled by an electric motor having an output shaft that regulates the phase relationship of the camshaft relative to the crankshaft. An electric motor controller can direct the electric motor regulating the cam phaser to change the phase between the camshaft and the crankshaft using a rotational position and/or speed of the output shaft. That is, depending on the design of the camshaft phaser, the electric motor can reduce or increase the rotational speed of the output shaft to thereby retard or advance the phase between the camshaft and the crankshaft. Apart from changing phase, the electric motor of the camshaft phaser may be directed by the electric motor controller to maintain a particular phase relationship between the camshaft and the crankshaft.
However, maintaining a phase relationship between the camshaft and the crankshaft using an electrically-actuated camshaft phaser may be challenging. A control system determines whether the camshaft phaser is maintaining a desired phase relationship between the camshaft and the crankshaft. Such a determination may involve receiving a position signal from a crankshaft sensor and a position signal from a camshaft signal and detecting, based on data from these signals, whether any angular drift exists between the camshaft and crankshaft and, if so, directing the electric motor to adjust the phase. Doing so involves maintaining the control system, sensors, and electric motor in an active state. It would be helpful to control the phase between the camshaft and the crankshaft without regard to the operational state of the control system, the sensors, and/or the electric motor of an electrically-actuated cam phaser.